1. Field of the Invention
This invention relates to a process for the removal of mercury from coal by way of microorganisms that oxidize iron, sulfur and other species binding mercury within the coal.
2. Description of the Related Art
It has been reported that emissions of mercury from coal-fired burners can be in the range of 0.5-22 lbs./trillion Btu and that the power-generating industry may emit about 50 tons of mercury each year, about a third of the total manmade emissions. It has also been suggested that there may be a plausible link between mercury emissions and mercury bioaccumulation in the food chain.
Current coal-fired power plants may not be required to have dedicated mercury removal equipment, and emissions control in the combustion of coal has traditionally been limited to the removal of mercury from off-gases. In post combustion mercury removal processes, the mercury exists at very low concentrations in a flue gas which is at very high temperatures that adversely affect the efficiency of mercury sorption technologies.
In 2003, the United States Environmental Protection Agency (EPA) suggested two approaches to reduce mercury emissions. In the first approach, emissions would be reduced from 48 to 34 tons/year by 2007 using existing technology. In the second approach, emissions would be reduced by 70% by 2018. Currently, there is a debate on the amount of mercury emission reduction, with stress by regulators to reduce emissions beyond what has been proposed by EPA. Needless to say, new technologies capable of reducing mercury emissions significantly will be needed in near future.
Mercury is naturally present in coal from different world sources and it has been reported that the concentration is typically in the range of 0.02-0.4 mg/kg. In the United States, coal from the Gulf Coast and Appalachian regions generally has the highest average concentration of mercury at 0.21-0.22 mg/kg. In a comprehensive review (Toole-O'Neil et al., Fuel 78:47-54, 1999), it was concluded that mercury in coal is most likely associated with the sulfur-containing iron compounds such as pyrite; however, a fraction of the mercury may be associated with the organic matter. It is expected that mercury and sulfur are closely associated in the coal as it is known that mercury sulfide is a low-solubility inorganic salt.
Various processes have been proposed for removing mercury from coal. U.S. Pat. No. 6,156,281 discloses a process for removing mercury and other trace elements from coal containing pyrite. A slurry of finely divided coal is formed in a liquid solvent capable of forming ions or radicals having a tendency to react with constituents of pyrite or to attack the bond between pyrite and coal and/or to react with mercury to form mercury vapors. The slurry is heated in a closed container to a temperature of at least 50° C. to produce vapors of the solvent. The vapors including solvent and mercury-containing vapors are withdrawn from the closed container, and then mercury is separated from the vapors withdrawn. Another example process is found in U.S. Pat. No. 5,403,365 which describes a process for producing low mercury coal wherein heated gas is used to drive off the mercury which is then collected.
Analysis of trace metals in coal is sometimes based on leaching the coal with dilute nitric acid. In unpublished studies, as much as 75% of the mercury could be removed through nitric acid leaching, and published results summarizing data from commercial cleaning facilities suggest that 12-78% removal is possible when pyrite is removed from coal via froth floatation. Use of a two-step hydrochloric acid wash process has also been demonstrated to leach mercury from coal up to 77%.
It is known that pyrite in coal can be utilized by members of the bacteria Acidithiobacillus (formerly Thiobacillus) ferrooxidans (A. ferrooxidans), and others which use both the reduced iron and sulfur in pyrite with the overall reaction:4 FeS2(s)+15 O2(g)+2H2O(l)-->2 Fe2(SO4)3(aq)+2H2SO4(aq).This reaction is stepwise beginning with the interaction between the pyrite surface and soluble Fe(III) to liberate elemental sulfur, S(0), and Fe(II). Fe(II) and S(0) are oxidized by the bacteria, yielding the overall reaction above. The reactions are carried out by the bacterium A. ferrooxidans or by two bacteria (A. ferrooxidans and A. thiooxidans) working together. The generation of sulfuric acid in the process lowers the pH and helps with further dissolution of pyrite, and will aid in the dissolution of mercury-sulfur compounds. The optimal pH for iron removal from coal pyrite was determined to be pH 2 in experiments with A. ferrooxidans (see Torma et al., Appl. Biochem. Biotechnol. 18:341-354, 1988). This pH is naturally obtained through the release of sulfuric acid by the bacteria. See also, U.S. Pat. Nos. 5,827,701 and 4,861,723, which are incorporated herein by reference along with all other patents and publications cited herein.
Another organism with good metal bioleaching capability is Leptospirillium ferrooxidans (L. ferrooxidans). This organism was found to comprise more than a 50% population in microbial species habitating biotopes such as mines and surrounding dump sites at temperatures above 20° C. Other reports also suggest the dominance of Leptospirillum genus in acid mine drainage environments. This is a strict chemolithoautrotroph, metabolizing ferrous iron and pyrite.
Microbial leaching for copper and uranium recovery has been used commercially for low-grade ore. Other metals including nickel, copper, and lead have also been studied for bioleaching potential using the same organisms. Other example biooxidation processes can be found in U.S. Pat. Nos. 6,383,458 and 5,007,620.
However, commercial technology for the precombustion removal of mercury from coal is not believed to be available at this time. Therefore, there is a need for a coal modification technique that will aid in the removal of mercury from coal prior to thermal processing.